Traditionally, many commercial applications of high temperature materials have been filled by Ni-,Co-, and Fe-based metal alloys. These materials function well at temperatures below about 800.degree. C., but rapidly lose strength upon exposure to higher temperatures. Thus, in the field of high temperature materials, researchers have focused on the use of heat resistant fibers to reinforce both metallic and ceramic materials. These high strength composites possess a unique combination of high strength, temperature stability, and low density. This allows for their use in materials for aerospace, automotive, and industrial applications.
Silicon-containing materials are known reinforcements for composite materials. These composites potentially possess high toughness levels and good performance characteristics, thereby making them highly suitable for applications which require light-weight structural materials having high elasticity, high strength, shapability, heat stability, electrical conductivity and heat conductivity. These composites are being increasingly investigated for structural applications.
It is known that many fiber-matrix combinations undergo extensive chemical reaction or interdiffusion between the fiber and matrix materials, each of which is likely chosen for the contribution of specific mechanical and/or physical properties to the resulting composite. Such reaction or interdiffusion can lead to serious degradation in strength, toughness, ductility, temperature stability and oxidation resistance. Some changes may result from the difference in the thermal expansion coefficients of the materials. SiC reinforcements are commonly fabricated by depositing SiC onto a refractory metal core material such as W, Ti, Mo, Ni, Fe, Ti, or NbTi. This core provides a template for SiC deposition and contributes to the high strength of the resulting reinforcement. In addition to the fiber-matrix chemical reactions described above, another potential source of degradation in strength of the reinforcement is the potential chemical reaction between the refractory core material and SiC. Typically, this chemical reaction occurs at 700.degree.-800.degree. C. and leads to the formation of brittle metal-carbide and metal-silicide compounds.
To compensate for these problems, a variety of coatings have been suggested for reinforcements intended for use in fiber-matrix composites. For example, U.S. Pat. No. 4,340,636 discloses a surface treatment for the formation of a carbon-rich coating on a stoichiometric SiC substrate filament. Similarly, U.S. Pat. No. 4,315,968 discloses coating SiC filaments with a coating of Si-rich SiC. U.S. Pat. No. 3,811,920 discusses applying a thin layer of TiC to a filamentary substrate having a SiC surface layer. Boron nitride has also been used as a SiC coating, as in U.S. Pat. No. 4,642,271.
Intermetallic matrix materials have experienced problems similar to those enumerated hereinabove when combined with reinforcements to produce high performance composites. The problems being experienced in this technology field are generally a result of the fact that the matrix material technology and fiber technology have evolved independent of one another, resulting in chemical and mechanical incompatibility of the precursor materials used to produce composites of the type disclosed hereinabove. The foregoing citations demonstrate various attempts within the field to overcome the inherent shortcomings of these composites by using coating materials to provide the needed characteristics or compatibility.
However, composite materials which have employed techniques and coatings such as the foregoing nonetheless remain limited for high temperature application by the chemical instability of the refractory core material in contact with sic.
Accordingly, an object of the subject invention is to provide a coating for a refractory metal core material which permits the use of SiC reinforcements in composite materials for use at high temperatures.
Another object of this invention is to provide a coating for a refractory metal core that prevents chemical reaction between the refractory metal core and SiC.
A further object is to provide a composite which contains SiC reinforcements which maintains high strength and toughness at high temperatures.